RE: RFC Remove classful causing incorrect routing behavior

[Mroczek, Joseph T]

> From: Andrey Borzenkov [mailto:address@hidden
> Sent: Monday, April 21, 2014 10:42 AM
> 
> В Mon, 21 Apr 2014 15:56:07 +0000
> "Mroczek, Joseph T" <address@hidden> пишет:
> >
> > > From: Behalf Of Vladimir 'f-coder/phcoder' Serbinenko On 19.04.2014
> > > 02:48, Mroczek, Joseph T wrote:
> > > > Hello:
> > > >
> > > > Currently, the DHCP logic assumes that if a gateway is received in
> > > > the DHCP
> > > packet the boot server is on a remote network. Given that CIDR is
> > > now over
> > > 20 years old, I think it is a safe assumption that a netmask will be
> > > offered in DHCP options.
> > > >
> > > > Can this be removed? Or is there still a need to cover the classful 
> > > > case?
> > > >
> > > Please detail the failure scenario.
> > > Current code follows standard behaviour for PXE clients and changing
> > > it would break any installation which relies on it.
> >
> 
> Hmm ... re-reading RFC2131 I ask myself what are conditions when
> *client* is supposed to use gateway_ip at all. According to RFC, giaddr is set
> by DHCP relay when it forwards request from client so server knows where
> to send reply to. DHCP relay then forwards reply on local subnet according to
> standard rules. RFC does not say anything about client usage of this field.
> Actually there is no requirement that DHCP relay also functions as normal
> router.

Use of giaddr as a gateway dates back to RFC951. At that point in time we did 
not have DHCP options, so this was a way to dynamicially learn the gateway.

> PXE spec indeed mentions giaddr as possible source for gateway to connect
> to TFTP server, without going into much details how stack selects between     
>  
> multiple gateways if present. But grub does not call PXE TFTP, right?

I have not seen any instances of it. All the tftp code I have seen uses tftp.c, 
which does not seem to leverage PXE TFTP functions. Just packet send/receive.
 
> > The failure will occur in most if not all cases where the DHCP sends a
> > gateway when the boot server is on the same subnet as the client,
> 
> In view of the above, how is it possible? DHCP server is not supposed to set
> this field at all - at the most it simply replies back to relay with same 
> value of
> giaddr it got. If giaddr is set it is already indication that server and 
> client are on
> different subnets.

For my case, DHCP server is on different subnet but boot/TFTP server is on same 
as client.

> >  with the additional condition that the gateway and boot server are on
> > different hosts. There may be some routers that will forward the
> > traffic back on the local subnet, but most routers do not do this. The
> > net result is grub fails to contact the boot server. I am not sure
> > what you mean by standard behavior. Mulitple Intel option ROMs (i386 &
> > EFI), broadcom option ROMs, gPXE, Microsoft WDS, and syslinux/pxelinux
> > all route properly for local boot servers when both netmask and
> > gateway are set.
> 
> >
> 
> 
> 
> > The only use cases I could see breaking with this change require two
> conditions, one of which I would be very surprised to see. The two conditions
> are that the boot server is on a remote network (not-common but does
> happen), and the DHCP server does not send a subnet mask in options (rare
> to non-existent.) Not sending a subnet mask in this day and age is arguably
> broken behavior.
> >
> > ~joe
> >
> >
> > > > Thank you for any attention you can pay this matter.
> > > >
> > > > ~joe
> > > >
> > > >
> > > > diff -Naur grub-2.02~beta2/grub-core/net/bootp.c
> > > > grub-2.02~beta2-jtm-
> > > clean/grub-core/net/bootp.c
> > > > --- grub-2.02~beta2/grub-core/net/bootp.c       2013-12-24
> > > 11:40:31.000000000 -0500
> > > > +++ grub-2.02~beta2-jtm-clean/grub-core/net/bootp.c     2014-04-18
> > > 20:38:05.858208600 -0400
> > > > @@ -191,18 +227,6 @@
> > > >    if (bp->gateway_ip)
> > > >      {
> > > >        grub_net_network_level_netaddress_t target;
> > > > -      grub_net_network_level_address_t gw;
> > > > -      char *rname;
> > > > -
> > > > -      target.type = GRUB_NET_NETWORK_LEVEL_PROTOCOL_IPV4;
> > > > -      target.ipv4.base = bp->server_ip;
> > > > -      target.ipv4.masksize = 32;
> > > > -      gw.type = GRUB_NET_NETWORK_LEVEL_PROTOCOL_IPV4;
> > > > -      gw.ipv4 = bp->gateway_ip;
> > > > -      rname = grub_xasprintf ("%s:gw", name);
> > > > -      if (rname)
> > > > -       grub_net_add_route_gw (rname, target, gw);
> > > > -      grub_free (rname);
> > > >
> > > >        target.type = GRUB_NET_NETWORK_LEVEL_PROTOCOL_IPV4;
> > > >        target.ipv4.base = bp->gateway_ip;
> > > >